Chemical etching of silicon materials is well-known as is indicated in U.S. Pat. No. 3,839,111 entitled "METHOD OF ETCHING SILICON OXIDE TO PRODUCE A TAPERED EDGE THEREON", Inventors: Edward John Ham and Ralph Robert Soden; U.S. Pat. No. 3,934,057 entitled "HIGH SENSITIVITY POSITIVE RESIST LAYERS AND MASK FORMATION PROCESS", Inventors: Wayne M. Moreau and Chiu H. Ting; and U.S. Pat. No. 3,737,314 entitled "MANUFACTURE OF PRINTING ELEMENTS BY A PHOTORESIST CHEMICAL ETCHING SYSTEM", Inventors: Robert L. Ruleff, William P. Lee II, Jay W. Childress, and Donald H. Knapke. These forementioned references teach both selective and non-selective etching for a variety of purposes in semiconductor fabrication.
Turning to the particular area of semiconductor pressure transducer fabrication, etching and other means of selective removal of silicon has also found wide usage. The semiconductor pressure sensor is typically formed as a diaphragm made of silicon material having a thickness dependent upon the measured pressure range. Stain sensitive resistors are diffused into the face of the diaphragm using known standard semiconductor techniques. The silicon diaphragm is also usually mounted to a support made of silicon or other material of suitable strength and compatible characteristics. A disadvantage of this usual type of structure is that very high stresses are generated at the bond area between the pressure sensitive diaphragm and its support base. The characteristics of the base material as well as the materials used to bond the diaphragm to the base can cause creep and instabilities when subjected to this stress. Clearly, what is needed is a diaphragm which will be thick and rigid and strong in the area of the mounting of the pressure sensitive diaphragm to the base and which is thin, flexible and sensitive to pressure in the area of the diffused resistors.
To achieve these mentioned results, some prior art pressure sensitive diaphragms were fabricated out of a single block of material to provide both the pressure sensitive area as well as the mounting base. No joint exists between the diaphragm and the mounting base to contribute to creep or instability. However, fabrication of this type of structure requires the forming of a hole into the silicon material which hole will be precisely located with respect to the diffused resistors on the other side of the diaphragm. The bottom of this hole should be substantially flat in the area of the diffused resistors with a good surface finish. Typical hole depths required vary from 5 to 10 mils to as much as 75 to 100 mils. A variety of fabrication methods have been used to form this hole. These methods include electro-discharge machining followed by an electro-chemical etch or polish, ultrasonic drilling or machining followed by electro-chemical etch or polish, and chemical or electro-chemical etching.
Electro-discharge machining and ultrasonic drilling are quite effective in forming the structure. However, they are complicated processes requiring elaborate equipment which can be very expensive. Further, these processes are subject to tool wear and require the two-step process, machining following by polishing or etching, to obtain the required surface finish.
The known chemical or electro-chemical etching involves usually the use of an etchant such as KOH:H.sub.2 O which may be used with silicon of the proper crystallographic orientation. The selective etch has different etch rates in different crystallographic directions which when combined with silicon of the proper orientation can etch a vertical wall. For etching circular holes, non-selective etchants (isotropic) are commonly used. The major problem with the isotropic etchant which is commonly a mixture of HF and HNO.sub.3 is masking to control the location of the etch. Photoresist is usually used for masking the KOH:H.sub.2 O etchant with very successful results. However, similar techniques for masking the isotropic etchants such as HF:HNO.sub.3 fail because of mask undercutting. Hence, selective etching with isotropic etchants such as HF:HNO.sub.3 has been difficult to do because of the lack of a suitable mask. To overcome this masking problem, various methods of electro-chemical etching have been developed, using weak solution of HF:HNO.sub.3 which can be successfully masked. The etch rate achieved, however, is very slow and not suitable for etching deep wall holes as required for the manufacture of pressure transducers having thin walls in the diffused resistor area and thick strong walls in the area mounted to the base.